A malaria parasite infected mosquito can inject approximately 100-200 Plasmodium sporozoites under the human skin during a blood meal. These sporozoites travel a considerable distance through several layers of tissue to reach the liver. The sporozoite journey from skin to the liver can take several minutes, during which it is exposed to the host immune system. Each successful sporozoite invasion can yield ˜30,000 blood stage merozoites, each capable of invading an RBC seconds after its release (Blum-Tirouvanziam, Servis et al. 1995). Hence immune interventions that block sporozoite invasion can be the most effective strategy to induce sterile immunity in humans (Blum-Tirouvanziam, Servis et al. 1995). The most abundant sporozoite surface protein of P. falciparum is the 397 amino acid long Circumsporozoite protein (CSP). A comparison of amino acid sequences of CSP across the genus Plasmodium revealed a highly conserved gene structure (Doolan, Saul et al. 1992). The central region of the gene consists of a species specific repeat sequence flanked by a N-terminal region that contains a conserved stretch of a five amino acid sequence called “region I” and the C-terminal that contains a conserved cell-adhesive motif similar to one found on thrombospondin.
The functional role of CSP in the life cycle of the parasite is multifaceted. Genetic knockout studies with CSP show its involvement in development of sporozoites from mosquito oocysts (Menard, Sultan et al. 1997). CSP also binds specifically to salivary glands and is involved in the movement of sporozoites from oocysts to the salivary glands (Wang, Fujioka et al. 2005) (Myung, Marshall et al. 2004). Genetic replacement of P. berghei CSP with the corresponding CSP from the avian malaria parasite P. gallinaceum showed a failure to invade mosquito salivary glands and to infect mice (Tewari R, Rathore D, et al. 2005). Once on the surface of the infective sporozoite stage, the N- and C-terminal regions of CSP have an adhesive function that along with the thrombospondin-related adhesive protein allow the sporozoite binding to heparan sulfate proteoglycans on the liver cell (Frevert 1999). CSP is also known to shield the sporozoite as it traverses through several layers of host tissues including professional phagocytes (Usynin, Klotz et al. 2007) and inhibits host cell protein synthesis. After invasion CSP gets exported into the hepatocyte cytoplasm and the nucleus where it can alter the gene expression profile of the host cell to protect the parasite and promote its growth and maturation (Singh, Buscaglia et al. 2007).
RTS,S is a human malaria vaccine grown in yeast cells and comprises the central repeats and the C-terminal cysteine rich region of Plasmodium falciparum CSP, fused to the S antigen of hepatitis B virus. The expressed protein self-assembles into a particle and is formulated with the proprietary adjuvant system ASOX (GlaxoSmithKline, Belgium) that contains immune stimulants MPL and QS21 (Cohen, Nussenzweig et al. 2010). Among the malaria naive individuals, RTS,S vaccination protects approximately 40% of vaccinees against experimental sporozoite challenge (Cohen, Nussenzweig et al. 2010). In a phase 2b trial the efficacy of RTS,S was estimated to be ˜35% against first clinical episode and ˜49% against severe malaria during an 18-month period among 1- to 4-year-old African children (Alonso, Sacarlal et al. 2004). In another phase II trial among 5-17 month old children, vaccine efficacy of RTS,S against clinical episodes was found to be 53% during an average 8 month period of observation (Bejon, Lusingu et al. 2008).